In a power distribution system, a high-voltage wire used to supply high-voltage power and a low-voltage wire used to supply low-voltage power are included. Standard voltages and allowable ranges to allow a deviation from the standard voltages are defined for the high-voltage wire and the low-voltage wire. For this reason, a voltage drop is calculated by dividing the power distribution system from the power substation to the load facility into a high-voltage system containing facilities such as the high-voltage wire and a switch and a low-voltage system containing facilities such as the low-voltage wire and a lead-in wire. Then, the voltage of the power distributed from the power substation is set such that a voltage before and after the voltage drop occurs remains in the allowable ranges of the high-voltage system and the low-voltage system, or the voltage of the power flowing through the power distribution system is adjusted by switching the switch disposed in the high-voltage system.    Patent Literature 1: Japanese Laid-open Patent Publication No. 10-187827    Patent Literature 2: Japanese Laid-open Patent Publication No. 2008-118799    Patent Literature 3: Japanese Examined Patent Application Publication No. 61-17221
However, in the above technology, the assumption that a reverse power flow from a distributed power source is not made so that the power exceeding a voltage allowable range in a power distribution system is not always prevented.
In other words, as a distributed power source such as photovoltaic power generation comes into wide use, the distributed power source is disposed in a customer in some cases. In this case, it may occur that the reverse power flows from the distributed power source disposed in the customer to the power distribution system of the electric power provider. Therefore, in a case where the power flows bidirectionally between the power substation and the load facility, the power levels at the respective facilities included in the low-voltage system may be significantly fluctuated due to the reverse power flow. For example, at the time when the reverse power flow occurs, even when the voltage falls within the allowable range, the load facility receives the distributed power for usage in a case where the reverse power flow does not occur. Therefore, the voltage in the power distribution system may be reduced out of the allowable range.